1. Field of the Invention
The present invention relates to a sheet processing device that performs a predetermined process on a conveyed sheet and an image forming apparatus including the sheet processing device integrally or separately.
2. Description of the Related Art
A sheet processing device capable of performing a finishing process on a sheet has been come into wide use in recent years. Specifically, the sheet processing device can perform various finishing processes, such as a punching process, an aligning process, a stapling process, a folding process, and a binding process, on a sheet conveyed from an image forming apparatus, i.e., a sheet on which an image is formed by the image forming apparatus. Incidentally, the sheet processing device can be built into the image forming apparatus, or provided separately from the image forming apparatus as an external device. However, the conventional sheet processing device has such a problem that when a stack of sheets to be stapled is discharged from a staple tray, a buckling distortion or a bending deformation may occur in the stack of sheets depending on a type and a size of the sheets, or a use environment. If a degree of the buckling distortion or the bending deformation is large, the stack of sheets may come in contact with a brush roller (a return roller) arranged on the upstream of the staple tray, and thereby causing a sheet jam. To avoid such a situation, in conventional technologies, a pressing unit is provided in the sheet processing device. The pressing unit presses on near a trailing end portion of the stack of sheets thereby aligning the sheets and also moving the stack of sheets not to come in contact with the brush roller.
A sheet processing device including such a pressing unit is disclosed in, for example, Japanese Patent No. 3748710. The sheet processing device disclosed in Japanese Patent No. 3748710 includes a staple tray, a trailing-end fence, a stapling unit, and the pressing unit. A sheet discharged from an image forming apparatus is stacked in the staple tray. An end of the sheet stacked in the staple tray in a sheet conveying direction is struck on the trailing-end fence thereby being aligned. The stapling unit staples a stack of sheets aligned by the trailing-end fence. The pressing unit is configured to be movable in a thickness direction of the stack of sheets stacked in the staple tray so as to change a distance between the pressing unit and a sheet-stacked surface of the staple tray. The stack of sheets is guided to the trailing-end fence while being pressed by the pressing unit.
However, in some of the conventional technologies, the pressing unit is fixed, i.e., the distance between the pressing unit and the sheet-stacked surface of the staple tray is kept constant regardless of the number of sheets staked on the staple tray. Therefore, it is possible to prevent the stack of sheets from having contact with the brush roller. However, it is not possible to reduce an occurrence of a buckling distortion or a bending deformation in the stack of sheets when the number of sheets is few. In this case, the stack of sheets passes by the pressing unit in a state where the sheets are still buckled or bent. To solve the problem, in the sheet processing device disclosed in Japanese Patent No. 3748710, the pressing unit is configured to be movable. However, the pressing unit does not move in consideration of a distance between a top-sheet face of the stack of sheets and the pressing unit, so that there is still a possibility of an occurrence of a buckling distortion or a bending deformation.
With an increase in processing speed of an image forming apparatus in recent years, there has been expected to provide a sheet processing device capable of processing at high speed. Therefore, it is necessary to improve the sheet processing device in such a manner that the stack of sheets is prevented from occurring a buckling distortion or a bending deformation and a wasted motion of the sheet processing device is reduced as much as possible to improve the productivity.